In mobile telecommunication systems of the time division multiple access (TDMA) type, time-division communication takes place on the radio path in successive TDMA frames, each of which consists of several time slots. In each time slot, a short information packet is sent as a radio frequency burst which has a finite duration and which consists of a set of modulated bits. The time slots are mainly used for transmitting control channels and traffic channels. On the traffic channels, speech and data are transmitted. On the control channels, signalling between a base station and mobile subscriber stations is carried out. An example of a TDMA radio system is the Pan-European mobile communication system GSM (Global System for Mobile Communications).
In conventional TDMA systems, each mobile station is assigned one traffic channel for data or speech transmission. Thus, the GSM system, for instance, may have up to eight parallel connections to different mobile stations on a same carrier wave. The maximum data transfer rate on one traffic channel is restricted to a relatively low level according to the available bandwidth and the channel coding and error correction used in the transmission, for example in the GSM system to 12 kbit/s, 6 kbit/s or 3.6 kbit/s.
A digital mobile communication system typically uses several connection types which can be divided into two categories: transparent and non-transparent connections. On a transparent connection, data is transferred through a traffic channel of the mobile communication system in a transparent way, which means that error correction on the radio path is carried out by employing channel coding only. In the GSM system the channel coding is Forward Error Correction (FEC). A non-transparent connection uses, in addition to channel coding, an additional protocol in which the data transmission over the traffic channel is repeated in case the data was not received correctly at the other end. In the GSM system, this communication protocol is Radio Link Protocol (RLP), used between a terminal adaptor of a mobile station MS and an interworking function IWF, which is typically at a mobile services switching center MSC. The RLP is a balanced (HDLC type) data transfer protocol having a frame structure. Error correction by the RLP is based on retransmission of frames corrupted on the traffic channel. There is another protocol, Layer 2 Relay (L2R), above the RLP. In the present patent application, the functional part of the TAF or IWF carrying out these protocols is referred to as an L2R/RLP unit.
In a normal data transfer state, the L2R/RLP unit packs user data into 200-bit long protocol data units (PDU), which are transmitted in 240-bit RLP frames over the radio interface to a second L2R/RLP unit. If there is no data or other information to be transferred between the two L2R/RLP units, discontinuous transmission (DTX) may be applied. DTX refers to a method reducing transmission on the radio path to a minimum (i.e. interrupt the transmission) during pauses in the data transfer. The aim is to reduce the power consumption of the transmitter, a very significant matter for the mobile stations, as well as the overall interference level on the radio path, which has an effect on the system capacity. The DTX operates independently for the uplink and downlink directions. The mobile communication network may either allow or prohibit the use of DTX.
In normal L2R/RLP operation, the PDUs are possibly filled only partially, because the application may limit the maximum user rate below the maximum rate on a traffic channel. The PDUs may be full if the actual user data rate on the terminal interface is high enough or if, due to delays caused by re-transmission or some other congestion, the L2R/RLP buffer has enough user data to fill one PDU completely. It is also possible, depending on the implementation, to prefer full PDUs to partially full PDUs. This can be accomplished by using a timer or a counter to slightly delay the building of a PDU until there is enough data available (e.g. in a buffer) for a full PDU to be built, or until the building of a PDU, albeit only a partially full PDU, cannot be delayed any longer. The timer may have a typical value in the order of 20 ms, in other words the repetition period of TDMA frames. The timer value should be relatively short in order not to introduce additional data transmission delays.
However, the data rates of present-day mobile communication networks are not sufficient for the new, high-speed data services. A solution is proposed for introducing higher data rates for mobile communication systems in the applicant's co-pending PCT application WO95/31878, unpublished on the priority date of the present application: two or more parallel traffic channels (subchannels) are used on the radio path for one high-speed data connection. The high-speed data signal is distributed to the parallel subchannels at the transmitting end for the transmission over the radio path, and then combined at the receiving end. In this manner, it is possible to provide data transfer services which, depending on the number of allocated traffic channels, have a transfer rate up to 8 times the conventional data rate. In the GSM system, for example, the total user data transfer rate 19.2 kbit/s is obtained with two parallel subchannels. This principle is also referred to as a multi-slot channel technique. High-speed data service thus obtained are referred to as HSCSD (High Speed Circuit Switched Data) services.
If the user data rate in a HSCSD service is lower than the maximum capacity of the radio link protocol, partially filled PDUs may be built and transmitted over the radio interface in the RLP frames. It is also possible that some subchannels carry full PDUs in their RLP frames and some subchannels occasionally carry partially full or empty PDUs in their RLP frames.
For the mobile station, this inefficient use of transmission capacity may lead to an unnecessarily high power consumption, heating of RF and other elements, and possibly a more complex scheduling of reception, transmission and neighbour cell monitoring than necessary for the actual user data rate.
For the radio interface, this leads to an increased interference, either in the uplink or the downlink direction, or both. For the base station and the IWF, reducing complexity is not that much of an issue as for the MS.
The prior art DTX implementation, and the slightly delayed building of PDUs can relief the situation somewhat but not completely. The DTX is mainly used mainly in cases when there is nothing at all to be transmitted. When some data has to be transmitted (at a low rate), the transmission requiring only a fraction of the allocated bandwidth, the conventional DTX does not suffice.